Uses of the 3D Cursor
Below are some uses of the 3d cursor in Blender with examples


Precise placement of new objects
Pivot for transformations, example - Transforms moving around the 3D cursor?
Pivot for modeling tools and modifiers (e.g. mirror, spin, screw...)
Moving the origin of objects (Ctrl-Alt-Shift-C), example - Change pivot or local origin of an object
A visual placeholder, for example to remember where something was previously in an animation.
Accurate relocation of objects/relocating objects to exactly the same place
Source location for the Clone Brush Tool (Ctrl+LMB to set).